I agree, OpenBSD has excellent security oriented APIs for writing programs in C. However I wouldn't rely too heavily on it if I were trying to make my software as portable as possible. Some security functions like malloc/calloc_conceal, pledge() and unveil() will appear in their man-pages but not on FreeBSD, Linux or Mac man-pages since they are not part of POSIX, so I would have to place #if defined(__OpenBSD__) on it.
For secure POSIX guidelines, I'd prefer OpenBSD's version over the Open Groups documentation [0].
[0] - http://pubs.opengroup.org/onlinepubs/9699919799/